This invention relates to motor driver circuits and, more particularly, to a driver circuit for driving a stepper motor wherein the driver circuit is responsive to a microprocessor.
Generally, stepper motors have increasingly been the prime mover of choice in preference over conventional DC motors for many applications. One reason for the increased employment of stepper motors is the ability to precisely control the displacement of a stepper motor without the need of employing elaborate servo-control systems.
Conventionally, a stepper motor advances in a step-wise fashion in response to actuation of a specific combination of internal switches, i.e., gating, thereby causing the stepper motor's output shaft to radially displace a discrete radial distance, i.e., step. Contiguous rotation is achieved by applying a prescribed gating sequence to the stepper motor. The stepper motor is also subjected to a given electrical current level relatable to the maximum torque output of the stepper motor, the speed of the stepper motor being a function of the time interval between steps.
Conventionally, the stepping sequence for an associated stepper motor is stored in a memory associated with the microprocessor which is in directing communication with a driver circuit. The driver circuit under the direction of the microprocessor gates the stepper motor to cause the desired rotational displacement, direction and speed of the stepper motor. The driver circuit is also in communication with a current/voltage source for directing current to the stepper motor under the direction of the microprocessor.
In certain applications, here of particular interest, in postage meter mailing machine applications, it is desirable for the stepper motor to drive a mechanism which must be rapidly accelerated to a desired speed. For example, in postage meter mailing machine application, a postage meter print drum assembly must be rapidly accelerated from a rest position to the required printing speed and subsquently decelerated to a stop between each print cycle. Rapid acceleration of the print drum assembly requires the stepper motor to deliver during initial displacements, i.e., acceleration of the print drum assembly, high amounts of torque in order to overcome sufficiently the inertial resistance of the print drum assembly in addition to driving a traversing mailpiece. Therefore, it is necessary to employ an upsized motor in order to achieve sufficiently high torque during print drum acceleration. However, the need for high torque represents only a small fraction of the stepper motor's operating cycle time. It is noted that because of spacial constraints, the use of large motors in such applications is undesirable. As a result, a compromise is present between motor size, promoting higher acceleration rates, and spacial constraints relative to desired postage meter mailing machine through-put. Further, large motors represent an increased cost factor.